Coherent gradient sensing (CGS) is an interferometric technique used to measure one or more shape-based properties (e.g., height, slope and curvature) of a surface. The CGS techniques utilize a collimated coherent optical beam as an optical probe to obtain shape information indicative of a specularly reflective surface formed of essentially any material. When the specularly reflective surface is not flat, the wavefront of the reflected probe beam is distorted and thereby the reflected probe beam acquires an optical path difference or phase change associated with the shape of the surface under measurement.
Two gratings spaced relative to each other are placed in the path of the reflected probe beam to manipulate the distorted wavefront for measurement. A first grating diffracts the reflected probe beam to spatially separate diffraction components of different orders. A second grating further diffracts each diffraction component produced by the first grating. An optical element (e.g., a lens) positioned relative to the second grating combines two selected diffraction components produced by the second grating by diffracting two different diffraction components produced by the first grating. The two selected diffraction components interfere with each other to produce an interference pattern. The diffraction by the two gratings effects a relative spatial displacement between the two selected diffraction components, referred to as the shearing distance. The interference patterns contain information that allows extraction of the discrete height differences between two locations on the reflective surface that are separated by the shearing distance. These height differences can be interpreted as local surface gradient, which in turn, can be further processed to obtain topography information through numerical integration or curvature information through numerical differentiation.
Despite the success of existing CGS systems, there is an increasing demand for CGS systems to provide improved measurements of a sample surface, including less measurement noise and greater resolution, in order to provide greater measurement functionality over a wider range of measurement applications.